A standard magnetic grab such as described in French patent document 2,441,577, German patent document 2,704,118 and European patent application 90,746 normally has two groups of permanent magnets each having a lower surface sitting atop a respective pole piece and an upper surface bridged by a piece of mild steel, forming in effect a downwardly pointing U-magnet. The magnets in one leg are polarized oppositely to those in the other leg so that one of the pole pieces is the north pole and the other the south pole of the magnet. A rotor situated between the legs of this symmetrical magnet can be rotated to juxtapose its north pole with the north-pole leg and its south pole with the south-pole leg in an active position to add together the magnetic fields and to make the grab capable of lifting a magnetically attractable object engaged with the pole pieces. When reversed, with the rotor north pole turned toward the south-pole leg and the rotor south pole turned toward the north-pole leg, all flux flows between the two legs through the rotor and the grab has no appreciable lifting capacity.
It has been suggested in U.S. Pat. No. 5,166,654 to use a pair of pole pieces each forming a downwardly directed pole and formed adjacent the respective poles with horizontally extending semicylindrical seats having approximately the same center of curvature and open concavely inward toward each other. One of the pieces has an upwardly directed upper surface to one side of the seats and the other piece has a lower surface directed downward at the upper surface and also to the one side of the seats. A rotor fitting in the seats is provided with a plurality of high-flux rotor magnets polarized about respective axes extending generally diametrically of the rotor axis. At least one low-flux permanent magnet having a vertical polarization axis between the upper and lower surfaces has its north pole engaging one of the surfaces and its south pole engaging the other surface. The rotor can be turned about its axis between an active position with the north poles of the rotor magnets turned toward the seat of the piece having the other surface and the south poles of the rotor magnets turned toward the seat of the piece having the one surface, and an inactive position with the north poles of the rotor magnets turned toward the seat of the piece having the one surface and the south poles of the rotor magnets turned toward the seat of the piece having the other surface.
Furthermore U.S. Pat. No. 4,465,993 has a pair of vertical and parallel pole plates between which is provided a cylindrical rotor that is rotatable to establish the active and inactive positions.
All these arrangements are fairly tall so that they have a high center of gravity and therefore can be fairly difficult to use. In addition they have a fairly small footprint so that lifting a large object with them requires that they be carefully centered on the object.
With today's powerful rare-earth permanent magnets it should in theory be possible to make a grab that is more powerful and lighter than the prior-art systems. Ideally such a grab would not need several sets of magnets. Nonetheless such an arrangement has not been made that constitutes a large improvement on the prior-art systems.